Co-Si-X aligned eutectics

ABSTRACT

Alloys of Co-Si-X, where X is selected from the group consisting of W, Mo or alloys thereof, are directionally solidified to produce aligned eutectics.

The invention described herein was made in the course of, or under, acontract with the Naval Air Development Center, Warminster, Pa.

The art has disclosed the unidirectional solidification of certain metalalloy systems to produce aligned eutectic structures. A eutectic alloyor composition is defined as one which upon being cooled from the liquidstate freezes or solidifies at a fixed temperature called the eutectictemperature to produce simultaneously a mixture of two or more types ofmetallurgical phases.

The present invention relates to the unidirectional solidification of acobalt-silicon-X alloy, where X is selected from the group consisting oftungsten, molybdenum or alloys thereof, to produce novel alignedeutectics consisting essentially of at least two phases. The presentaligned eutectics are substantially or completely free of dendrites andhave substantial tensile strength at elevated temperatures of the orderof 900°C making them useful for a wide range of applications.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying figures in which:

FIG. 1 is a photomicrograph (300 X) of a longitudinal section, growthdirection vertical, of a unidirectionally solidified aligned Co-Si-Weutectic of the present invention.

FIG. 2 is a photomicrograph (2500 X) of a transverse section of theCo-Si-W aligned eutectic of FIG. 1 showing three phases. Gray lamellaeare Co₂ Si, white lamellae are Co-rich solid solution, and third phaseis Co₃ W₂ Si.

Briefly stated, the present process comprises forming a Co-Si-X alloybody where X is selected from the group consisting of W, Mo or alloysthereof and where the amount of Si ranges from 21 atomic % to 27 atomic% and X ranges from 1 atomic % to 4 atomic %, said alloy having aeutectic temperature at which it solidifies to form a solid alloy havingat least two phases, establishing a liquid-solid interface in said alloybody, establishing a thermal gradient ranging from 100°C/cm to 500°C/cmin the liquid at said liquid-solid interface, and unidirectionallysolidifying said liquid at said liquid-solid interface at a rate rangingfrom 0.5 cm per hr to 12 cm per hr.

The aligned eutectic of the present invention is substantially orcompletely free of dendrites and consists essentially of at least twophases in the form of alternating lamellae significantly parallel toeach other and to the thermal gradient. These phases have an averageinterlamellar spacing ranging from about 2 microns to 20 microns and aresubstantially uniformly distributed throughout the aligned eutectic.

In the Co-Si-X alloy of the present invention X is selected from thegroup consisting of W, Mo or alloys thereof. Specifically, the presentCo-Si-X alloy contains Si in an amount ranging from 21 atomic % to 27atomic % and X in an amount ranging from 1 atomic % to 4 atomic %. Alloycompositions outside these ranges do not produce the present alignedeutectics. When X is W, the preferred alloy contains 24 atomic % Si and2 atomic % W because it produces microstructures free or most free ofdendrites.

Generally, in carrying out the instant process, the alloy components aremelted together to obtain as uniform a molten mass as possible. Themolten mass is then cast by a conventional method to produce an alloybody of the desired size.

The alloy body can be directionally solidified by a number ofconventional methods. Generally, the apparatus is comprised of a heatedvertical mold provided with a cooling system at its lower end, usuallywater, and means for controlling the rate of solidification, usually bypulling the ingot-containing mold at a constant rate away from the heatsource used to melt the ingot.

Directional solidification is carried out under an inert atmosphere suchas argon. Specifically, in carrying out the directional solidification,the alloy body is heated above its eutectic temperature, which generallyis about 1200°C, to liquify at least a portion thereof throughout itsentire cross-sectional area to establish a liquid-solid interface.

In preparing the aligned eutectic alloy of the present invention, thethermal gradient in the liquid at the liquid-solid interface and therate of solidification are controlled during unidirectionalsolidification. Specifically, this thermal gradient is the change intemperature in the liquid per centimeter of length in the liquid phaseimmediately in front of the liquid-solid interface and ranges from 100°Cper cm to about 500°C per cm. The specific thermal gradient used dependslargely on the specific rate of unidirectional solidification and tosome extent on alloy composition.

The unidirectional rate of solidification, i.e. the number ofcentimeters of aligned eutectic formed in one hour, ranges from 0.5 cmper hr to 12 cm per hr. Faster rates of solidification requirecorrespondingly higher thermal gradients to maintain significantalignment of the lamellar phases of the present aligned eutectic. Also,the faster the rate of solidification, the finer is the averageinterlamellar spacing which, in the present invention, ranges from 2microns to 20 microns.

The aligned Co-Si-X eutectic alloy of the present invention issubstantially or completely free of dendrites and consists essentiallyof at least two phases in the form of alternating lamellae significantlyor substantially parallel to each other and distributed substantiallyuniformly throughout the aligned eutectic. One of the lamellar phases isa Co-rich solid solution phase containing Si in solution, generally inan amount ranging from 8 to 15 atomic %, and may contain X in solutionranging up to the total amount present. The second lamellar phase is aCo₂ Si phase or a Co₂ Si phase containing X in a minor amount.Generally, each of said lamellar phases is significantly orsubstantially uniform throughout the aligned eutectic. The presentaligned eutectic has an average interlamellar spacing ranging from 2microns to 20 microns with the finer spacing leading to higher tensilestrength but decreased thermal stability. Optimum interlamellar spacingis determined by the particular application and may vary from oneapplication to another. Also, the present aligned eutectic has a minimumtensile strength at 900°C of at least 40,000 psi measured in an inertatmosphere such as argon or a substantial vacuum.

In the Co-Si-X alloy, when X is W, the resulting aligned eutectic of thepresent invention also contains a third phase which is rod-like in formand which is composed of Co₃ W₂ Si. This third phase is located at theinterfaces of the lamellar phases and extends longitudinally through thealigned eutectic. Also, it is distributed substantially uniformlythroughout the aligned eutectic.

In one embodiment of the present invention, chromium is substituted inan amount ranging up to 18 atomic % for a portion of the Co in theCo-Si-X alloy to produce an aligned eutectic with improved oxidation andcorrosion resistance. The chromium has no significant deleterious effecton the microstructure of the resulting aligned eutectic and generallysubstantially all of it is present in solid solution in the Co-richlamellae. The resulting chromium-containing aligned eutectic also has aminimum tensile strength at 900°C of at least 40,000 psi measured in aninert atmosphere such as argon or a substantial vacuum. Amounts ofchromium in excess of 18 atomic % tend to have a deleterious effect onthe tensile properties of the aligned eutectic.

The present aligned eutectics are particularly useful as hightemperature structural alloys such as blades or vanes for hightemperature turbines. Specifically, in addition to their substantialtensile strength, the aligned eutectics have a microstructure which isthermally stable, i.e., the microstructure undergoes no significantchange in an inert atmosphere such as argon or a substantial vacuum at atemperature up to and including 1100°C. Also, the present alignedeutectics have a surface which is significantly stable, i.e.,significantly resistant to oxidation, because of the protective natureof the silicon.

The invention is further illustrated by the following examples:

EXAMPLE 1

A 74 atomic % Co, 24 atomic % Si, 2 atomic % W alloy was formed underargon and cast into a rod 3/8 inch in diameter and 5 inches long.

Conventional apparatus was used to directionally solidify the rod. Itincluded a high frequency induction vertical furnace, a water-cooledcopper chill plate and a lowering device. Specifically, the rod wasplaced in a close-fitting alumina crucible and the resulting assemblyplaced in the vertical furnace. Directional solidification was carriedout under an argon atmosphere. A portion of the rod was meltedthroughout an entire cross-sectional area and a liquid-solid interfacewas established at the bottom portion of the rod. A thermal gradient of750°C/in or 300°C/cm was established in the liquid at the liquid-solidinterface. The liquid at the liquid-solid interface was thendirectionally solidified at a rate of 1/4 inch per hr or 0.625 cm perhr.

The resulting aligned rod was sectioned longitudinally and transverselyand examined using standard metallographic procedure. It had asignificantly uniform aligned eutectic microstructure as illustrated inFIGS. 1 and 2 and appeared to be free of dendrites. Specifically, it hadthree phases, two of which were in the form of alternating lamellaesignificantly parallel to each other and to the thermal gradient and hadan average interlamellar spacing of 8 microns. One of the lamellarphases was a Co-rich phase containing Si and W in solid solution and thesecond lamellar phase was a Co₂ Si phase. The third phase was rod-likein form and was located at the interfaces of the lamellar phases asillustrated by FIG. 2. The third phase was determined to be Co₃ W₂ Si.All of the phases were distributed substantially uniformly in thealigned eutectic.

EXAMPLE 2

An aligned Co-Si-W eutectic rod was prepared as set forth in Example 1.Its tensile properties were determined by a standard technique at atemperature of 900°C in vacuum and are given in Table I.

EXAMPLE 3

An aligned eutectic rod was prepared as set forth in Example 1 exceptthat the alloy composition in atomic % was 68.3 Co, 21.6 Si, 7.4 Cr, 2.7W.

A portion of the aligned rod was sectioned longitudinally andtransversely and examined using standard metallographic procedure. Ithad a significantly uniform eutectic microstructure and appeared to befree of dendrites.

The tensile properties of the remaining portion of the aligned rod weredetermined in the same manner set forth in Example 2 and are given inTable I.

                                      TABLE I                                     __________________________________________________________________________    TENSILE PROPERTIES AT 900°C                                                               0.1 pct                                                                             Ultimate                                                                Yield Tensile                                                                             Elong-                                            Composition     Stress                                                                              Strength                                                                            ation                                          Ex.                                                                              (at pct)        (10.sup.3 psi)                                                                      (10.sup.3 psi)                                                                      (pct)                                          __________________________________________________________________________    2  74Co, 24Si, 2W  58.8  64.7  8                                              3  68.3Co, 21.6Si, 7.4Cr, 2.7W                                                                   76.3  79.2  6                                              __________________________________________________________________________

Table I shows that the aligned eutectics of the present invention havesatisfactory tensile properties at a temperature of 900°C making themuseful for a wide variety of high temperature structural applicationssuch as blades or vanes for high-temperature turbines.

The following cited copending patent applications are, by reference,made part of the disclosure of the present application.

In copending U.S. Pat. application, Ser. No. 609,593 entitled "Co-Si-XAligned Eutectics" filed of even date herewith in the name of James D.Livingston and assigned to the assignee hereof, there is disclosedalloys of Co-Si-X, where X is selected from the group consisting of Ta,Nb, V or alloys thereof, which are directionally solidified to producealigned eutectics.

In copending U.S. Pat. application, Ser. No. 609,591 entitled "Co-Si-XAligned Eutectics" filed of even date herewith in the name of James D.Livingston and assigned to the assignee hereof, there is disclosedalloys of Co-Si-X, where X is selected from the group consisting of Al,Ga or alloys thereof, which are directionally solidified to producealigned eutectics.

What is claimed is:
 1. A process for producing an aligned solid eutectichaving significant tensile strength which comprises forming a Co-Si-Xalloy body where X is selected from the group consisting of W, Mo oralloys thereof and where the amount of Si ranges from 21 atomic % to 27atomic % and X ranges from 1 atomic % to 4 atomic %, said alloy having aeutectic temperature at which it solidifies to form a solid alloy havingat least two phases, establishing a liquid-solid interface in said alloybody, establishing a thermal gradient ranging from 100°C/cm to 500°C/cmin the liquid at said liquid-solid interface, and unidirectionallysolidifying said liquid at said liquid-solid interface at a rate rangingfrom 0.5 cm per hr to 12 cm per hr, said aligned eutectic beingsubstantially or completely free of dendrites and consisting essentiallyof at least two phases in the form of alternating lamellae significantlyparallel to each other and to the thermal gradient and having an averageinterlamellar spacing ranging from about 2 microns to 20 microns, saidphases being substantially uniformly distributed throughout said alignedeutectic.
 2. A process for producing an aligned solid eutectic havingsignificant tensile strength according to claim 1 where X is W andwherein there is also a third rod-like phase extending longitudinallythrough said aligned eutectic at the interfaces between said lamellarphases.
 3. A process for producing an aligned solid eutectic accordingto claim 1 where chromium is substituted in an amount ranging up to 18atomic % for a portion of the cobalt in said Co-Si-X alloy.
 4. Aunidirectionally solidified aligned eutectic of a Co-Si-X alloy havingsignificant tensile strength where Si ranges from 21 atomic % to 27atomic % and where X ranges from 1 atomic % to 4 atomic %, said X beingselected from the group consisting of W, Mo or alloys thereof, saidaligned solid eutectic consisting essentially of at least two phases inthe form of substantially alternating lamellae significantly parallel toeach other and substantially uniformly distributed throughout saidaligned eutectic having an average interlamellar spacing ranging fromabout 2 microns to 20 microns, said aligned eutectic being substantiallyor completely free of dendrites.
 5. A unidirectionally solidifiedaligned eutectic of a Co-Si-X alloy according to claim 4 where X is Wand where there is also a third rod-like phase extending longitudinallythrough said aligned eutectic at the interfaces between said lamellarphases.
 6. A unidirectionally solidified aligned eutectic according toclaim 4 wherein chromium is substituted in an amount ranging up to 18atomic % for a portion of the cobalt in said Co-Si-X alloy.